1. Field of the Invention
The present invention relates to the field of detecting flaws and discontinuities in materials, and more particularly, to the detection of flaws and discontinuities in materials using magneto-optic visualization and eddy current excitation.
2. Art Background
The present application is related to U.S. Pat. No. 4,755,752, issued July 5, 1988 and U.S. Pat. No. 4,625,167 issued Nov. 25, 1986.
In many scientific, engineering and manufacturing applications, near surface cracks, voids, discontinuities and flaws in electrically conducting materials must be detected in order to insure the structural integrity of a material. For example, the material integrity of components comprising many air and space vehicles is critical for their proper operation, especially with regard to high stress components such as turbine and fan blades, rocket engine systems, air frames, etc.
A number of techniques have been developed and utilized in order to detect cracks, flaws, or the like, in such materials. For example, magnetic particle methods have been employed which utilize static or "low" frequency (less than 100 Hz) magnetic having field components parallel to the surface of ferromagnetic alloys, which may be induced by currents paralleling these surfaces. The parallel surface currents in may be induced, either directly, by contact electrodes, or indirectly, using coils or other arrangements of carrying conductors surrounding or adjacent to target material, and low frequency excitation. Magnetic fields paralleling the surfaces of the target material are distorted by cracks or near surface flaws and these portions may be detected through the use of a magnetic powder deposited on the material. Various types of powders have been developed for the of sub-surface flaws. Each magnetic particle in these typically consists of a single magnetic domain (i.e. region of essentially uniform magnetization). magnetic powder is applied dry or in a wet to a target material where a crack or flaw is present, the magnetic particles tend to aggregate and form a bridge regions of field nonuniformities which are a with the flaw. By mixing various pigments, dyes, and the like, with the magnetic powder, the cracks or flaws are rendered visible.
Although the magnetic powder technique is widely employed, it is a dirty and time consuming method which requires the induction of large surface currents or large applied fields in the material under study. Magnetic particle methods are best suited for use with low frequencies and ferromagnetic alloys. The inertial properties of magnetic particles renders the magnetic powder techniques ineffective when high frequencies are used. Moreover, magnetic particles do not work on non-ferromagnetic materials such as aluminum and other alloys used in the aerospace industry.
Another method which has been utilized in order to detect flaws or cracks in non-ferromagnetic materials is the "eddy" current technique. Eddy current techniques typically utilize a time varying electromagnetic field which is applied to the target material being examined. Non-contact coils are used to excite eddy currents in the target material, such that these currents tend to flow around flaws and result in field distortions which allow the flaw to be detected in a number of well known ways. For example, circuit parameters characterizing the mutual interaction between the exciting coil and the responding target material may comprise the parameters of capacitance, inductance or reactance. However, eddy current techniques require a considerable amount of support equipment and most techniques do not result in a flaw image but rather produce data from which flaw information can be obtained only after appropriate analysis has been completed.
As will be described, the present invention provides a method for the direct visualization of surface and near surface cracks, flaws, etc. in non-ferromagnetic and ferromagnetic conducting materials. The present invention provides direct visualization of the dynamic magnetic fields associated with the various flaws or other discontinuities in a target material, and overcomes the disadvantages associated with prior art material flaw imaging methods. In addition, the present invention is compatible with the requirements of eddy current methods while producing images of flaws directly, without the additional support equipment and data analysis required by most eddy current systems.